NMR or MRI
In MRI systems or nuclear magnetic resonance (NMR) systems, a static magnetic field B0 is applied to the body under investigation to define an equilibrium axis of magnetic alignment in the region of the body under investigation. An RF field is applied in the region being examined in a direction orthogonal to the static field direction, to excite magnetic resonance in the region, and resulting RF signals are detected and processed. Generally, the resulting RF signals are detected by RF coil arrangements placed close to the body. See, for example, U.S. Pat. No. 4,411,270 to Damadian and U.S. Pat. No. 4,793,356 to Misic et al. Typically, such RF coils are either surface type coils or volume type coils, depending on the particular application. Normally separate RF coils are used for excitation and detection, but the same coil or array of coils may be used for both purposes.
Generally, solenoid, saddle or Alderman-Grant (see, e.g., Alderman, D. W. and Grant, D. M. J. Magn. Reson. 36:447 (1979)) type linear volume RF coils were used for NMR imaging. A further increase in S/N can be realized with the use of quadrature coils over the conventional linear coil designs. See, for example, U.S. Pat. No. 4,467,282 to Siebold and U.S. Pat. No. 4,707,664 to Fehn; see also, U.S. Pat. No. 4,783,641 to Hayes and U.S. Pat. No. 4,751,464 to Bridges, for highly homogeneous, quadrature volume coils commonly referred to as the birdcage and transverse electromagnetic wave (TEM) resonators respectively, in the NMR community. A four-ring birdcage of Murphy-Boesch (see, e.g., U.S. Pat. No. 5,21,450) was introduced to further improve the S/N and B field uniformity along the coil axis.
The recent introduction of phased array radar and ultrasound technology to NMR by Roemer (see, e.g., U.S. Pat. No. 4,825,162) has allowed the use of multiple RF coils in an effort to increase image S/N and resolution. It is noted that multiple RF coils arranged in unique configurations can be used to reduce the time spent by the patient inside the MR scanner. This is extremely useful in reducing patient claustrophobia and discomfort. This aspect is also useful in increasing patient throughput in a MR scanner. U.S. Pat. No. 5,258,717 to Misic and U.S. Pat. No. 5,646,531 to Renz applied the above array concept to further enhance S/N of the volume birdcage coil. A novel technique was introduced by Wang [7] to reduce the coupling between neighboring surface coils in an effort to maximize overall S/N. (See, e.g., Wang, Jianmin. “A Novel Method to Reduce the Signal Coupling of Surface Coils for MRI”, ISMRM 4th Scientific Meeting, Book of Abstracts, page 1434, 1996). More recently, SMASH and SENSE were introduced to further enhance the S/N and imaging resolution and reduce the scan time. These techniques mandate the use of multiple coils in an array configuration over the imaging FOV. (See, e.g., Klaas P. Pruessmann, et al., “SENSE: Sensitivity Encoding for Fast MRI”, Magnetic Resonance in Medicine 42:952-962 (1999), and Sodickson D K, et al., “Simultaneous acquisition of spatial harmonics (SMASH); ultra-fast imaging with radio-frequency coil arrays”, Magnetic Resonance in Medicine 1997; 38:591-603).
A strong need met by the present invention is to enhance further the signal-to-noise (S/N) of an RF coil receiver system, which will make it possible to obtain higher resolution images needed for accurate diagnosis. With the improved S/N, one can reduce the scan time thereby reduce patient discomfort and concomitantly increase patient throughput in a MR scanner. Conventional coils may be summarized as follows:
Solenoid
The solenoid RF coil looks very similar to that of the conventional solenoid. A two turn solenoid RF coil with two identical value C1 tuning capacitors in series is shown in FIG. 10. The inductance of the coil turns along with C1 resonate the coil near the NMR frequency. By adjusting the tuning capacitors C1 and matching the coil across C1 (A) with impedance matching networks (not shown), one can derive the homogeneous mode which can be used to image at the NMR frequency. The B field orientation of this homogeneous mode is along the coil axis, in the Z direction. The points where the dotted line intersects the coil are at virtual ground, they have no net potential (shown with“x” marks).
Alderman-Grant Design
A simple schematic of a Alderman-Grant type resonator is shown in FIG. 11. The coil consists of two end rings connected by two straight segments. Tuning was accomplished with C1 whereas, matching was accomplished across C1 (A) with impedance networks (not shown) similar to the solenoid explained above. Here currents in the two end rings were 180 degrees out-of-phase. Useful mode for NMR imaging was oriented along the Y axis. This design was suited for cylindrical volumetric applications, e.g., imaging the human head, whole body, knee, wrist etc.
For the symmetric coil of FIG. 11, the virtual ground plane existed in the central transverse plane at the coil center (see dotted line of FIG. 11). The points where the dotted line intercepted the straight segments were at virtual ground, that is these two points (shown with x) had no net potential.
TEM Resonator
An extension of the cavity resonator well suited for volumetric NMR imaging applications is the transmission line TEM resonator of Bridges. (See, e.g., James F. Bridges. “Cavity Resonator with Improved Magnetic Field Uniformity for High Frequency Operation for High Frequency Operation and Reduced Dielectric Heating in NMR Imaging Devices”, U.S. Pat. No. 4,751,464, dated Jun. 14, 1988, and specifically FIG. 1 thereof). This design consisted of a number of transmission line segments originating and terminating on a common shield, which served as a ground and cavity wherein all of the flux was contained. A TEM section is shown in the present application in FIG. 12. The dark section depicts the RF shield and the region between the straight segment and the closed shield encompasses a cavity. The cavity was resonant with C1. Coupling to the cavity resonator can be accomplished with inductive pickup loops, one such rectangular loop is shown in FIG. 12. A total of two such loops may be used to drive the coil in quadrature. See the aforementioned reference U.S. Pat. No. 4,751,464 for details on the coupling and drive mechanisms for the TEM resonator.
For a symmetric cavity of FIG. 12, the central point of the transmission line segment i.e., the point where the dotted line intercepted the straight segment was at no net potential. Such was the case for the central points of all of the transmission line segments, they were at virtual ground. Thus the virtual ground plane for the symmetric cavity resonator was located in the central transverse plane (entire cavity resonator not shown).
It is noted for the above case of the solenoid, Alderman-Grant and TEM resonators, details of the coupling and driving mechanisms are not shown. It is hereby envisaged that those skilled in the art can easily understand and successfully build these designs by reading RF coil articles mentioned in the references.
Birdcage
The birdcage coil is well known in the art and is shown in FIG. 1a. The coil comprises of two rings connected by several straight segments referred to as legs. This coil has several resonance modes, of interest is the principal k=1 mode for homogeneous imaging. The principal mode has two linear modes, oriented orthogonal to one another. The outputs from these modes can be combined using analog circuitry or digitally combined in the receiver system. The birdcage provided a 41% improvement in S/N and expended one-half the power over the conventional linear coil.
In addition, owing to the sinusoidal currents in the coil periphery the birdcage provided a highly homogeneous B field in the transverse planes (XY) inside the coil, ideal for volume imaging (whole-body, head, knee, wrist, etc.). The B field profile along the coil axis however mimicked a gaussian distribution, with maximum at the coil center. See FIG. 1b. Note, the birdcage can be designed to operate in the low-pass, high-pass, band-pass or band-stop configurations.
The dotted line of FIG. 1a is the virtual ground plane for this symmetric birdcage. The points where this plane intersects the coil legs are at virtual ground, that is at no net potential. For a birdcage with eight fold symmetry, the points of virtual ground are at the center of the legs. This can be evidenced by a finger touch method, the tuning and matching of the coil will remain virtually unaltered.
For a complete theory of the birdcage, reference is made to Tropp, James, “The Theory of the Birdcage Resonator”, Journal of Magnetic Resonance 82, 51-62, 1989. For different methods of coupling to the birdcage, see U.S. Pat. No. 4,887,039 to Roemer. An alternative coupling method using a series tuned inductive loop is shown in FIG. 1a. Normally, two such loops are needed to drive the two principal linear modes that are oriented orthogonal to one another.
Four-Ring Birdcage
The four-ring of Murphy-Boesch was introduced to improve the S/N and RF homogeneity of the birdcage along the coil axis. Here, two outer birdcages were coupled to each other via interconnecting segments as represented in FIG. 2a. Since a large fraction of currents traversed the inner interconnecting segments, a homogeneous B field distribution was realized along the coil axis. However, little or no S/N improvement was seen at the coil center as represented in FIG. 2b. 
2 Birdcage Array The two birdcage array of Misic addressed this deficit to an extent as represented in FIG. 3a. Here, two birdcages placed sequentially were partially overlapped to cancel their net mutual flux coupling. Individual modes from the two coils were routed to separate channels of the MR system. The coil system provided some improvement in S/N at the coil overlap and over the imaging field-of-view (FOV). This coil setup also displays a guassian style B field profile along the coil axis as shown in FIG. 3b. 
3 Birdcage Array
Likewise in the array design of Renz, three quadrature birdcage coils were overlapped to cancel the net mutual flux as represented in FIGS. 4a and 4b. This array system also provided improved S/N at the coil overlap with some improvement over the imaging FOV.
It is noted when overlapping neighboring volume coils fine coil movement adjustments are necessary to achieve critical coupling, where the net mutual flux coupling between coils is set very close to zero. In addition, canceling the coupling between the co-linear modes in the neighboring birdcages in this fashion, does not satisfy the critical coupling for orthogonal modes in neighboring coils. That is in the case of the two birdcages as represented in FIG. 3a, achieving critical coupling between 1a and 2a (or between 1b and 2b) does not satisfy the criteria for critical coupling between 1a and 2b (or between 1b and 2a). Additional circuitry or mechanisms are warranted to rectify this situation. Misic utilized a complicated design scheme, which was cumbersome to optimize individual coils in the array (mode tuning, matching, alignment and isolation) which in turn made manufacturing an arduous process.
Accordingly, a strong need met by the present invention is an RF coil system, wherein significant S/N improvements can be achieved over the entire FOV. Further, the present invention provides a coil system that can be manufactured with relative ease when compared to geometrically isolated volume coils.
Note, the above volume coil designs (solenoid, saddle, Alderman-Grant, TEM, birdcage) were routinely used in MRI. The features of the present invention can be readily applied to the above and other volume coils designs (such as other cavity and non-cavity designs, counter-rotating coil CRC pair, helmholtz pair, saddle, etc.).